The disclosure relates generally to porous cellular structures and more particularly to fused silica based porous cellular structures.
Demand and consumption of fossil fuels whether it be for transportation, manufacturing activities, or generation of electricity is growing steadily worldwide. The increasing demand outstrips supply and drives up cost of energy, and the emissions from combustion of fossil fuel degrade the environment and adversely affect human health. In the short term, nitric oxide, nitrous oxide, hydrocarbons, carbon monoxide, and soot drive the formation of smog and acid rain. Longer term, the otherwise inert carbon dioxide associated with burning of a carbon-based fuel accelerates global warming. In response, governments around the world are imposing ever tighter regulations on acceptable levels of pollutants. The measures being implemented control mobile and stationary sources of emissions from industrial to commercial and consumer activities.
In the transportation industry, regulations and fuel costs have fostered the development of more fuel efficient and cleaner engines, as well as more capable aftertreatment systems. Acceptable levels of NOx and soot (PM) originating from diesel and gasoline powered engines have been reduced numerous times over the past two decades in North America and Europe for light and heavy duty vehicle classes. Regulations not too different from these are being considered for phased implementation in China, India, Russia, and Brazil. The favored aftertreatment system for use with diesel engines includes a diesel oxidation catalyst (DOC), a soot filter, and a system for selective catalytic reduction (SCR) of NOx. These systems are just now being implemented on a massive scale. Each component utilizes a cellular ceramic either as a catalyst support or as the basis of the filtering structure.
The cellular ceramic substrates that support the SCR and DOC catalysts were originally developed in the 1970's to use as the catalytic converter for treatment of exhaust gases from gasoline powered passenger cars. The environment in the exhaust system of a gasoline powered vehicle from that era was especially severe. Cordierite became the preferred material to perform in this environment as it withstands extremely high temperatures that can melt most materials, severe thermal shock conditions associated with initial start-up of a vehicle, and transitions between heavy to low load conditions that will cause cracking in most materials. It can also tolerate fuel containing high concentrations of sulfur.
The situation some thirty years later looks quite different. Rapid responding sensors and control systems allow vehicles to automatically self-adjust and respond when malfunctions do occur to prevent damage. Temperature excursions that would cause melting are rare in the aftertreatment system of gasoline powered vehicles. Temperatures in diesel exhaust systems are also much lower than those in gasoline. The maximum temperature is no higher and likely much less than 1100° C. Fuel sulfur concentrations have been reduced from more than 500 ppm to less than 50 ppm to enhance catalyst lifetime and reduce acid rain. The requirements that the substrate and filter materials are resist attack by acid condensates such as from oxides of sulfur are not as severe.
The process for manufacture of cordierite substrates and filters is energy intensive and the source of greenhouse gas emissions. Temperatures of more than 1400° C. are used to drive reactive sintering to form cordierite. The firing process is also time consuming. This is because the heating cycles are slow to limit strains due to thermal gradients and shrinkage mismatches are required to prevent cracking. For all of these reasons, there is an opportunity for new materials that can be manufactured in a less energy intensive way to be used as catalyst supports in diesel and gasoline aftertreatment systems. Furthermore, such cellular substrates can also find application as membrane supports for chemical separations, filtration of water streams to purify water or separate desired chemical or biological products in industrial scale processes.